Evolution of massive binary black holes

Qingjuan Yu

Princeton University

July 21, 2002

Outline

• Introduction• Evolution of massive binary black holes (BBHs)• Possible observational characteristics of

surviving BBHs• Summary

Introduction: Most galaxies house massive black holes (BHs) at their centers

• suggested by QSO energetics and demography (e.g. Soltan 1982, Rees 1984)

• observations: massive dark objects in nearby galactic centers (e.g. Kormendy & Richstone 1995, Magorrian et al. 1998)

Quasar PKS 2349 (HST) M87 (HST)

Galactic center

NGC 4258

(Tremaine et al. 2002)

Introduction: Questions

• Is it possible that the massive BHs in some galactic centers are binary black holes (BBHs) (e.g. by galaxy mergers, Begelman, Blandford & Rees 1980)– How do BBHs evolve? (separation as a function of time)– Do BBHs merge or how long can they survive?

(e.g. comparison with a Hubble time)

• Orbital properties of surviving BBHs? Possible observational characteristics of surviving BBHs? (appropriate methods to probe BBHs?)

Introduction: Why interesting?

• BBHs provide a laboratory to study BH physics.

LISA: BBH mergers gravitational waves test for gravitation theory or stimulus for new physics. BBH merger rates?

• Understanding galaxy formation

– the M• – and M• –L correlations a close link between the formation and evolution of galaxies and their central BHs. (e.g. Tremaine et al. 2002)

– a probe of the hierarchical model

Laser Interferometer Space Antenna (LISA)

Evolution of massive BBHs

Evolution of massive BBHs

1. Dynamical friction stage

m

Mr

Nt sun

82

c1-

c6

df

10

pc 100s km 200log

yr104

decreasing a10-5pc

incr

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1010yr

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Dynamical friction

Evolution of massive BBHs2. Non-hard binary stage

3. Hard binary stage

dynamical friction (two-body interactions) and three-body interactions with stars passing in their vicinity

bound

decreasing a10-5pc

incr

easi

ng

1010yr

10kpc

Dynamical friction

three-body interactions with low-J stars;

-1 (E: BBH energy)

(Heggie 1975)

(Quinlan 1996)

Evolution of massive BBHs

4. Gravitational radiation stage

decreasing a10-5pc

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1010yr

10kpc

Dynamical friction

Gravitational radiation

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pc 01.0yr108.5)(

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(Peters 1964)

Evolution of massive BBHs

• Main uncertainty is in the non-hard binary stage and the hard binary stage..

Are low-J stars depleted before the gravitational radiation stage?

• Analogy: stellar tidal disruption rates around massive BHs. (e.g. Magorrian & Tremaine 1999).

With the depletion of initial low- J stars, consider the refilling by two-body relaxation and tidal forces in the host galaxy.

decreasing a10-5pc

incr

easi

ng1010yr

10kpc

Dynamical friction

Gravitational radiation

bottleneck

decreasing a10-5pc

incr

easi

ng1010yr

10kpc

Dynamical friction

Gravitational radiation

• Sample: nearby early-type galaxies observed by HST (Faber et al. 1997)

• Depends on BH masses, and velocity dispersions and shapes of host galaxies– small BHs (m2/m1<10-3) do not

decay into galactic centers;– BBHs are more likely to have

merged in low-dispersion galaxies and survive in high-dispersion galaxies;

– BBHs are more likely to have merged in highly flattened or triaxial galaxies and survive in spherical and nearly spherical galaxies

• Estimated orbital properties of surviving BBHs: – separation: 10-3 –10 pc

BBH evolution in realistic galaxy models (Yu 2002):

increasing velocity dispersion

increasing flattening

merged BBHs

surviving BBHs

merged BBHs

surviving BBHsincreasing triaxiality

Possible observational characteristics of surviving BBHs

• double nuclei (upper limit ~ HST resolution)• bending or wiggling of jets

(e.g. Blandford, Begelman, Rees 1980)• double-peaked emission lines from broad line regions

associated with BBHs in active galactic nuclei (AGNs) (Gaskell 1996)

• periodic behavior in the radio, optical, X-ray or -ray light curves (e.g. Valtaoja et al. 2000, Rieger & Mannheim 2000)

• broad asymmetric Iron K emission line shape from a two-accretion-disc system associated with a BBH (Yu & Lu 2001)

Fe K lines: a tool to probe BBHs in AGNs?

• Strongest lines of evidence for the existence of massive BHs– Broad and asymmetric

(Doppler and gravitational broadening)

– Short-term variability (~104s)– Emitted from inner disc region– Profiles are affected by the

inclination between the observer and the disc.

• Two-accretion-disc system associated with a BBH with different spin axis directions

Fe K line profile

(Yu & Lu 2001)

Summary• The orbital evolution of BBHs depends on the velocity

dispersion and shape of the host galaxy, and on the masses of BHs.

• BBHs are most likely to survive in spherical or nearly spherical and high-velocity dispersion galaxies. The upper limit of the separations of surviving BBHs is close to the HST resolution for the typical galaxies in the study. The absence of double nuclei in the centers of nearby galaxies does not mean that they have no BBHs.

• If all galaxies are highly triaxial, there will be no surviving BBHs.

• Possible observational characteristics of surviving BBHs (e.g. Iron K line profile from two accretion discs with different inclinations).